1. Field of the Invention
The present invention generally relates to heat exchanger tubing. The present invention also generally relates to corrugated tubing having textured internal and external surfaces. The corrugated tubing may have linear or helical corrugations.
2. Description of Related Art
A heat exchanger tube may be used in a process that transfers heat between a first fluid inside the heat exchanger tube and a second fluid outside of the heat exchanger tube. The efficiency of heat transfer between the first fluid and the second fluid may be a complicated function that depends on the characteristics of the fluids, on the characteristics of the heat exchanger tube, and on the characteristics of fluid movement relative to the heat exchanger tube. The term xe2x80x9cfluidxe2x80x9d refers to a liquid, a gas, or a combination of a liquid and a gas. A heat exchanger tube may also be used to transfer heat between a fluid and a solid. The solid may be located inside or outside of the tube.
Each end of a tube may be pointed. A pointed tube may have reduced diameter cylindrical portions at each end of the tube that transition to a larger diameter main body section of the tube. Pointed tube ends may facilitate attachment of the tube to support structures. The support structures may be tube sheets of a heat exchanger. Tube sheets may support several tubes within a shell of a tube-and-shell heat exchanger. Fluid that is directed past outside surfaces of tubes of a tube-and-shell heat exchanger may flow in a direction that is substantially coaxial to a longitudinal axis of the shell of the heat exchanger. Tubes having pointed ends may be easier to position and seal to support structures than are tubes that do not have pointed ends. U.S. Pat. No. 5,311,661, which issued to Zifferer and which is incorporated by reference as if fully set forth herein, describes an apparatus that may be used to form heat exchanger tubes having pointed ends.
It is desirable to maximize the heat transfer rate across a wall of a tube of a heat exchanger. Increasing the surface area of a tube may increase the heat transfer rate across the tube. Also, directing fluid flow past and through a tube in desired fluid flow patterns may increase the heat transfer rate across the tube.
One method of increasing the surface area of a tube is to attach fins to an outer surface of the tube. Fins may be attached to a tube after the tube is formed, or fins may be formed in the outer surface of the tube. Fins may be formed on the outer surface of a tube by a finning tool of a finning machine. A finning tool typically includes three or four disks mounted on an arbor. The disks form a spiraled flight of fins on an outer surface of a tube during use. The fins formed by a finning tool may have heights that are greater than about 30 mils (0.030 inches). Generally, the fins formed by a finning tool are oriented substantially perpendicular to the longitudinal axis of the tube. A small amount of skew from a true perpendicular orientation allows the finning tool to provide a driving force to the tube that moves the tube through the finning machine.
Fins may be oriented substantially perpendicular to a longitudinal axis of the tube, or the fins may be oriented substantially parallel to the longitudinal axis of the tube. Fins on an outer surface of a tube that are substantially perpendicular to a longitudinal axis of the tube may be used in heat transfer applications where fluid flow is directed substantially perpendicular to the longitudinal axis of the tube. Heat exchanger tubes of condensers and evaporators may be finned tubes wherein the fins are oriented substantially perpendicular to longitudinal axes of the tubes. Fins that are oriented substantially parallel to a longitudinal axis of a tube may be used in heat transfer applications where fluid flow is directed substantially coaxial to the longitudinal axis of the tube. Tubes having fins that are oriented substantially parallel to longitudinal axes of the tubes may be used in tube and shell heat exchangers.
Fins on an outer surface of a tube may promote the development of areas that have little or no fluid movement when fluid flows by the tube. Such areas may develop on a side of a fin that is opposite to a direction of fluid flow past the tube if the fins of the tube are not oriented to allow fluid to flow adjacent to the tube. Such stagnant areas may decrease the heat transfer efficiency of a tube. Such stagnant areas may promote charring or thermal degradation of a heat transfer fluid.
Another method of increasing the surface area of a heat exchanger tube is to texture the inner surface of the tube. A knurling tool may be used to form a groove and rib pattern on an inner surface of a tube. The knurling tool may be placed within the tube. Force may be applied to an outer surface of the tube to press the inner surface of the tube against the knurling tool. Pressing the inner surface of the tube against the knurling tool forms a knurl pattern on the inner surface of the tube.
A finning tool and a knurling tool may be used in combination to form a tube that has a finned outer surface and a knurled inner surface. U.S. Pat. No. 4,886,830, which issued to Zohler and which is incorporated by reference as if fully set forth herein, describes a method of forming a tube that has a finned outer surface and a knurled inner surface.
An alternate method of texturing a tube is to form a desired pattern of ribs and grooves on surfaces of a flat metal plate. The plate may then be rolled into a cylindrical shape. A weld may be formed to join the ends of the plate together and form a tube. U.S. Pat. No. 5,388,329, which issued to Randlett et al., describes a method of manufacturing an extended surface heat exchanger tube using a rolled and welded metal plate.
Another method that may be used to increase the surface area of a tube is to corrugate or convolute the tube. The corrugations may be linear corrugations or helical corrugations. Linear corrugations may be formed in a tube by passing the tube through a corrugating die. The corrugating die may have angularly spaced die teeth that are positioned and shaped to progressively indent the wall of the tube at equally spaced points around the tube. U.S. Pat. No. 5,311,661 describes a system for forming linearly corrugated heat exchanger tubing.
Helical corrugations or convolutions may be formed in a tube by passing the tube through a corrugating die. A die and machinery used to produce a helically corrugated tube may be substantially the same as shown in U.S. Pat. Nos. 4,377,083, which issued to Dale et al.; 4,514,997, which issued to Zifferer; 5,409,057, which issued to Zifferer; and 5,551,504, which issued to Zifferer. Each of these patents is incorporated by reference as if fully set forth herein. Another method of forming helical corrugations in a heat exchanger tube is to heat and twist the tube as described in U.S. Pat. No. 4,437,329, which issued to Geppelt et al.
A heat transfer rate across a tube may be increased by directing fluid flow in a desired flow patterns through and by the tube. A desired flow pattern may increase internal mixing of a heat exchange fluid. A desired flow pattern may promote non-laminar fluid flow of one or both of the heat exchange fluids that flow by and through the tube. In a straight, smooth-walled cylindrical tube, fluid may flow past or through the tube in a laminar flow pattern. Laminar fluid flow may develop a boundary layer at a wall of the heat exchanger tube. The boundary layer may inhibit heat transfer throughout the fluid. Non-laminar fluid flow may minimize the formation of a boundary layer and promote internal mixing of the fluid so that heat transfer takes place throughout the fluid.
One method that may be used to obtain a desired fluid flow pattern is to change the geometrical configuration of the surfaces of a heat exchanger tube. The geometrical configuration of the surfaces of a heat exchanger tube may be changed by texturing the surfaces of the tube. Texturing the surfaces of the tube may increase the heat transfer surface area of the tube and promote internal mixing of fluid that flows through or by the tube.
Another method that may be used to obtain a desired fluid flow pattern is to corrugate the tube. The corrugations may be linear corrugations or helical corrugations. Linear corrugations may significantly alter the configuration of a tube so that non-laminar fluid flow is obtained for fluid flowing through and by the linearly corrugated tube. Helical corrugations may also significantly alter the configuration of a tube so that non-laminar fluid flow is obtained for fluid flowing through and by the helically corrugated tube. A helically corrugated tube may cause angular fluid flow by and through the tube. The angular fluid flow may cause internal mixing of the fluids flowing by and through the tube.
A corrugated heat exchanger element having textured inner and outer surfaces may be formed. The corrugations may be helical or linear corrugations. The texturing of the inner and outer surfaces may be patterns of grooves formed in the surfaces of the heat exchanger element. The heat exchanger element may have extended surface area. The heat exchanger element may also have surface features that result in desired flow patterns around and through the element. The extended surface area and surface features may provide improved heat transfer characteristics for a heat exchanger that includes textured and corrugated heat exchanger elements.
Inner and outer surfaces of a tube may be simultaneously textured with a texturing machine. The texturing machine may include an outer knurling device and an inner knurling device. The knurling devices may be used to form grooves in the inner and outer surfaces of a tube. Ribs may be formed in the tube surfaces between adjacent grooves. Heights of the ribs formed by the knurling devices may be less than about 35 mils (0.035 inches), and are preferably less than about 20 mils. The height of the ribs may be greater than about 4 mils. Heights have been expressed in terms of heights of the ribs, but the heights could also be expressed in terms of the depth of the grooves. For example, the depths of the grooves may range from about 35 mils to about 4 mils. The ribs formed in the outer surface of the tube may have a different height and a different pattern than the ribs formed in the inner surface of the tube. The ribs and grooves formed in the surfaces of the tube may increase the surface area of the tube, promote internal mixing of fluid that flows by or through the tube, and inhibit formation of stagnant areas of fluid adjacent to inner and outer surfaces of the tube.
The grooves and ribs may be formed in a helical pattern about a longitudinal axis of the tube. Texturing on an outer surface of a tube may be formed in a helical pattern by a texturing machine. An angle of the pattern relative to a longitudinal axis of the tube may be less than 90xc2x0, and is preferably less than about 45xc2x0. The angle of the pattern relative to the longitudinal axis of the tube is preferably greater than about 2xc2x0. Texturing on an inner surface of the tube may also be formed in a helical pattern. An angle of the inner tube surface pattern relative to a longitudinal axis of the tube may be less than about 90xc2x0, and may preferably be between about 5xc2x0 and 45xc2x0, and may more preferably be about 30xc2x0. The patterns of ribs and grooves in the inner and outer surfaces of a tube may be formed at angles less than 45xc2x0 so that the tube may be used as a heat exchanger element wherein fluid flows by and through the tube in directions that are substantially coaxial to the longitudinal axis of the tube.
An embodiment of a texturing machine may be used to form an angle pattern in an outer surface of a tube that is oriented in an opposite direction to an angle of a pattern formed in an inner surface of the tube. For example, a pattern formed in an outer surface of a tube may be a 20xc2x0 right-hand helical pattern of ribs and grooves, while a pattern formed in an inner surface of the tube may be a 30xc2x0 left-hand helical pattern of ribs and grooves. In an alternate embodiment, the pattern orientation in the outer tube surface may be formed in a left-hand helical pattern, and the pattern orientation in the inner tube surface may be formed in a right-hand helical pattern. The oppositely oriented patterns may cause the formation of a cross-knurled pattern in the outer and inner surfaces of the tube. The cross-knurled pattern may be a result of grooves being formed in the outer surface when ribs are formed on the inner surface. Similarly, grooves may be formed in the inner surface when ribs are formed on the outer surface. Embodiments of texturing machines may form helical patterns in tubing that are in the same orientation. For example, a helical pattern in inner and outer tube surfaces may be oriented in a right-hand helical pattern. A helical pattern in inner and outer tube surfaces may also be oriented in a left-hand helical pattern.
A tube that is to be textured by a texturing machine may be placed over a mandrel of the machine so that a portion of a first end of the tube extends beyond the outer knurling device. The outer knurling device may be pressed against the tube to press an inner surface of the tube against the inner knurling device. A drive or drives may be engaged to move the tube through the machine so that the knurling devices form textured inner and outer tube surfaces. The drive or drives may be disengaged before the outer knurling device reaches a second end of the tube. Placing a portion of the first end of the tube beyond the outer knurling device and disengaging the knurling machine before reaching the second end of the tube leaves un-textured portions of tubing at each end of the tube. Un-textured portions of tube may allow the tube to be easily attached and sealed to support structures. The support structures may be tube sheets of a heat exchanger.
Each end of a textured tube may be pointed by a pointing machine to promote easy attachment of the tube to support structures. To point an end of a tube, the end of the tube may be brought into contact with a tube-pointing die. The tube-pointing die may form a frustro-conical section and a reduced diameter, cylindrical section.
A corrugating machine may corrugate a textured tube. In one embodiment, a corrugating machine forms linear corrugations in a textured tube to produce a heat exchanger element. In another embodiment, a corrugating machine forms helical corrugations in a textured tube to produce a heat exchanger element. A corrugating machine may form 3 to 20 corrugations in a textured tube that initially started as a 1 xc2xd inch diameter un-textured tube. Preferably, a corrugating machine forms 4 to 8 corrugations in a textured tube. A corrugating machine may include a corrugating die and a tube driving mechanism. A corrugating die of a helically corrugating machine may be rotatively mounted within the corrugating machine. The drive mechanism, which may include two independent units, may initially drive a tube into a corrugating die, and then pull the tube through the die.
An advantage of a corrugated and textured heat exchanger element may be that the element has extended surface area. Both the corrugations and the texturing of the inner and outer surfaces of the heat exchanger element may increase the surface area of the element. Another advantage of a corrugated and textured heat exchanger element may be that the textured surfaces and the corrugations promote desired fluid flow patterns through and by the element. The texturing on the outer and inner surfaces of the heat exchanger element may promote internal mixing of fluid that flows adjacent to the element. The internal mixing of the fluids may inhibit fouling and plugging within and adjacent to the heat exchanger element. Corrugations in a heat exchanger element may significantly change the fluid passageways through and by the element so that non-laminar fluid flow patterns develop within the element even at relatively low fluid flow rates through the element.
Another advantage of a textured and corrugated heat exchanger element may be that the element includes un-textured, reduced diameter, cylindrical portions at each end of the element. The un-textured and cylindrical portions may allow the heat exchanger element to be easily and conveniently sealed to support structures. The support structures may be tube sheets of a heat exchanger. A heat exchanger element may be sealed to a support structure by a sealing method. Sealing methods include, but are not limited to, welding or application of sealant. Attaching a heat exchanger element that has un-textured ends to a support structure may be easier to accomplish than attaching an element with textured ends because special measures do not have to be implemented to ensure that a seal is formed adjacent to each groove of the texturing in the element.
Another advantage of a textured and corrugated heat exchanger element may be that the corrugations and the formation of cylindrical portions at the ends of the element result in an element that has increased structural strength as compared to a cylindrical tube. The increased structural strength may inhibit bending and deformation of the heat exchanger element during assembly of the element into a heat exchanger. Other advantages of a textured and corrugated heat exchanger element may include that the element is sturdy, durable, simple, efficient, reliable and inexpensive; yet the heat exchanger element is also easy to manufacture, install, maintain and use.